Serum haemolytic classical and alternative pathways of complement in infancy: age-related changes.

The haemolytic activity of complement was evaluated in the serum of healthy children from birth to 2 years of age using the kinetic method for the determination of the time needed to lyse 50% of target red cells (t 1/2). No sex-linked differences were observed in any of the age groups studied and the lowest lytic activity levels for both complement pathways were detected in neonates. The two pathways, however, showed different maturation patterns, i.e., lytic activity levels similar to those of adults were reached between the 1st and 3rd month of life (classical pathway) and around the 13th month (alternative pathway). In the age group of 7 to 24 months, the lytic activity of the classical pathway was higher than in adults. The present data permitted us to establish normal ranges of t 1/2 values for the classical and alternative pathways in serum of healthy neonates and children aged 1 to 24 months.